1. Field of the Invention
The present invention relates to a microwave filter, particularly to a switchable frequency response microwave filter.
2. Description of the Related Art
The filter plays an important role in wireless communication. When the frequency of a signal is at the bandpass region of the filter, the signal is allowed to pass. When the frequency of a signal is at the bandstop region of the filter, the signal is attenuated. In other words, the filter controls the response of a communication system around a certain frequency.
Generally, filters are classified into high pass filters, low pass filters, bandpass filters and bandstop filters, which respectively have different circuit architectures. Therefore, only via adjusting bandwidth or changing the center frequency can signal attenuation be achieved in a single circuit architecture. However, circuit designers sometimes cannot attain the desired filtered signal merely via adjusting bandwidth or changing the center frequency but have to use filters of other circuit architectures. For example, a bandpass filter allows medium-frequency signals to pass but intercepts high-frequency signals and low-frequency signals. It is impossible for a bandpass filter to intercept medium-frequency signals but allow high-frequency signals and low-frequency signals to pass because high-frequency signals and low-frequency signals have opposite frequency response in a bandpass filter. When two different frequency responses are needed, two independent filter structures are usually adopted, and a control circuit is used to shift the signal path from a filter structure to another filter structure. However, such a design has the disadvantages of a complicated circuit and an increased circuit area.
For overcoming the abovementioned conventional problems, the present invention proposes a switchable frequency response microwave filter, which can switch between a bandpass frequency response and a bandstop frequency response, wherein totally replacing the circuit architecture is unnecessary, and the complexity of the conventional circuit is reduced, and the circuit area is decreased.